Transmitters are well known and widely used in the electronics industry. Within the communication field, Transmitters are used in a variety of applications. Such applications include, for example, wireline communications such as PCI express, SATA and HDMI, and optical communications such as SONET and GPON. Transmitters are typically designed to perform within a given set of boundary conditions and to perform according to a specified standard. Typical conditions include, for example, performance over operating temperature ranges, sensitivity to supply noise, and the like. Typical performance standards include, for example, Output signal jitter generation, output signal rise and fall time, output impedance, and the like.
A conventional HDMI transceiver is shown in FIG. 1. It consists of a transmitter 101 DC coupled to a receiver 102 through a transmission line 103. HDMI at low data rates does not require any termination at the transmitter side. However, at higher data rates the reflection degrades the performance, and the source termination 104 is needed.
FIG. 2 shows a typical piece of a conventional programmable resistor used in differential circuits including HDMI transmitter. It consists of 2 identical resistors 201 and 202 and a P transistor 203 as a switch. A programmable termination resistor consists of several of these pieces or branches in parallel. When the voltage of the gate of the P transistor in one branch is high the transistor is OFF, and it is like an open circuit, and consequently it does not have any impact on the total resistance. However, if the voltage of the gate is low, the transistor is ON, and the resistance of the branch will be equal to 2R+rds where rds is the Drain-Source resistance of the transistor. In this scheme the bulk connection of the transistor is normally connected to the supply voltage (VDD) or the common mode voltage of the output as shown in FIG. 3. Resistors 204 and 205 are large so they do not load the output.
In an HDMI application the supply voltage of the termination resistors at the receiver is 3.3V+/−5%. However, at the transmitter side the supply voltage might be smaller, e.g. 2.5V or 1.8V. The traditional scheme can still work as long as the devices used in the transmitter can tolerate 3.3V. However, if the devices cannot tolerate 3.3V, a special technique must be used to make sure the devices are not under stress. For example if P transistor 203 is a 1.8V device, the gate of the transistor cannot be connected to ground because the Gate-source or gate-drain voltage of a 1.8V device cannot be more than 1.8V+(˜20%) in order to meet the required lifetime.
To solve this problem a technique was introduced in US20110096848 A1. FIG. 4 shows this technique in a simplified form. In this scheme the P transistor turns on or off through switch 403. The key point here is when the switch is on the low voltage at the gate of transistor 203 is not zero and it is around Vbias. Vbias is a constant voltage generated by a bandgap reference, or the like. Therefore, transistor 203 does not go under stress. Transistor 404 is used to protect the switch 403 from stress which is a conventional method. In this scheme Vref is a voltage proportional to common mode voltage of the output to make the range of the protection wider.
Although the above technique protects the devices from stress, it causes some inaccuracy in making calibrated termination resistor. The inaccuracy comes from the fact that since Vbias is fixed, any changes in the common mode voltage of the output due to change in the supply voltage or output current or the like, will cause changes in the Vgs and hence the rds of transistor 203. Since rds is not negligible and Vgs might change by several hundred mV, the inaccuracy might be several percentages.